Ocean Thermal Energy Conversion

Ocean Thermal Energy Conversion or OTEC generates electricity using the difference in temperature of seawater.  The oceans cover 70 percent of the Earth’s surface which makes them the world’s largest solar collector.  OTEC uses the difference of warm tropical shallow water and cold deep water to run a heat engine.  The temperature between the warm and cold seawater needs a difference of 36 degrees to produce a significant amount of power.   This temperature difference exists between deep and shallow waters within 20 degress of the equator.

A tremendous amount of solar energy is absorbed by the oceans everyday.  If only a small fraction of this energy is converted into electricity it could supply more than twenty times the amount of electricity consumed by the U.S. everyday.   OTEC is a vast renewable resource with the potential to produce billions of watts of electricity.  The energy available is more than other renewable ocean resources like wave power but the extraction of energy is expensive and only has the efficiency of 2%.

A heat engine is a commonly used to create energy from heat.  A heat engine uses a high heat temperature reservoir and a low temperature reservoir.  As heat flows from one reservoir to another this energy is captured.  This captured energy is then used to turn turbines which generate electricity. 

OTEC plans require large expensive intake pipes which are submerged a mile or more into the ocean where the cold water is pumped to the surface.   There are three basic systems used.  The Closed cycle system takes warm surface seawater and pumps it through a heat exchanger where it vaporizes a low boiling point liquid.  The expanding vapor turns a turbine which generates electricity.  The cold seawater is pumped through a second heat exchanger where the vapor is condensed and reused in the system. 

Open cycle OTEC takes warm seawaters and subjects it to low pressure causing it to boil.  The expanding steam turns a low pressure turbine which generates electricity.  The steam which is almost fresh water is then condensed by passing it through the cold seawater. 

Hybrid OTEC uses both open cycle and close cycle systems.  Warm seawater enters a vacuum chamber and is flash evaporated into steam.  The heat from the steam vaporizes a low boiling port fluid that drives a turbine which generates electricity. 

By products of an Open or Hybrid OTEC system include fresh water from seawater.  Cold seawater from the OTEC plant can be used for air conditioning.   The nutrient rich cold seawater can feed salmon and lobster. 

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